Colour changing composition and colouring polymeric articles made therefrom

ABSTRACT

A process is provided for producing a polymeric layer having a desired image thereon or a three dimensional article comprising a number of such layers in which a layer of a liquid photocurable composition which comprises photo colourable particles is cured by light and selected areas thereof are irradiated with light of a different dose, thereby forming the desired image which composition comprises particles dispersed in it which are micro-capsules containing a photosensitive colour changing composition within a barrier layer which is substantially impermeable to the components of the colour changing composition or are solid particles comprising an immobilised photosensitive colour changing composition. The process permits the use of the same initiating mechanism to cure the resin and to change the colour of the particles.

[0001] This invention relates to colouring polymeric articles andmaterials for use therein.

[0002] It is known from our patent specification WO 97/09168 to colourregions of a polymeric layer or three dimensional polymeric article bycuring a layer of a polymeric composition by exposing it to light, andirradiating a region of the layer with light of a different dose therebyeffecting colour formation in the said region. Three dimensionalarticles may be produced by repeating the process on successive layers.A similar disclosure is given in U.S. Pat. No. 5,514,519.

[0003] Problems may arise in such a process however if the initiatingmechanism for curing the resin and forming the colour is the same, forexample if both are initiated by a free radical or ionic (e.g. acationic) mechanism, as in this case the whole layer or article may becoloured to some extent. This can arise if the initiating system whichis activated to cure the resin produces free radicals or ions capable ofinitiating colour formation; even if they are relatively inefficient ininitiating colour formation the result may be aesthetically undesirableor if a block of resin containing a three dimensional coloured image isto be formed any coloration of the non-imaged parts of the block may atleast partly obscure the image. If an image which comprises two or morecolours is desired it may be difficult to provide enough differentinitiating systems to permit each colour to be formed independently ofeach other's colour and free from interference from the polymerisationinitiator of the resin.

[0004] This invention comprises a process for producing a polymericlayer having a desired image thereon or a three dimensional articlecomprising a number of such layers in which a layer of a liquidphotocurable composition which comprises photo colourable particles iscured by light and selected areas thereof are irradiated with light of adifferent dose, thereby forming the desired image which compositioncomprises particles dispersed in it which are micro-capsules containinga photosensitive colour changing composition within a barrier layerwhich is substantially impermeable to the components of the colourchanging composition or are solid particles comprising an immobilisedphotosensitive colour changing composition. The barrier or solid stateof the particles will also act to limit the passage of initiatingspecies into the micro-capsule.

[0005] In this invention the initiation mechanism may be the same ordifferent, but it is particularly suitable for cases in which thecomposition is both cured and coloured by cationic initiators; forexample, epoxy resins may be so cured in the presence of micro-capsulescontaining cationically initiated colouring agents.

[0006] The colour changing composition suitably comprises a photoinitiator and a colour former which becomes coloured, changes colour orbecomes more intensely coloured when the photoinitiator is activated. Itmay alternatively become bleached.

[0007] If it is desired to use an initiator which is sensitive to thelight used for curing, the particles, for example the barrier and/orcontents of the microcapsule may comprise a light absorbing substancewhich absorbs at least part of such light. Initiation of full colourchange will then require a greater dose of light in intensity orduration than is needed for curing.

[0008] The barrier of the microcapsule may be permeable to materialsother than the components of the colour forming composition and it maybe desirable in manufacture to treat the microcapsules with a substance,for example ammonia, to decolourise the contents.

[0009] The particles may if desired contain a small quantity of a baseor other material to maintain them in a colourless state until athreshold level of radiation is encountered. This may suitably beintroduced by using a barrier which is permeable to the base, which maybe a lower (e.g. C₁₋₂) amine or preferably ammonia and exposing themicrocapsules to the base. If they are produced in a coloured orslightly coloured condition this procedure may also enable them to bedecolourised as aforesaid.

[0010] The dose of light may differ in intensity, duration orwavelength. For convenience and cost it may be preferred to carry outcure and colour changing using a common light source (UV lamp or UVlaser such as He—Cd, Argon Ion, YAG etc.), for example a laser which maysuitably be traversed at a slower rate over the area to be coloured.

[0011] The invention also comprises particles which comprise animmobilised photosensitive colour changing agent which are preferablymicro-capsules which may be as previously described which comprise abarrier layer enclosing a photosensitive colour changing composition,the barrier layer being substantially impermeable to the components ofthe said composition. “Immobilised” means not capable of migratingoutside the particle. Such particles may for example be included inpreparations for protecting the skin from sun and be adapted to changecolour, for example to red when a suitable limit of exposure to sunlighthas been received. They may also be used in monitoring the exposure tolight of substrates to be tested for resistance to light, or theexposure of patients to light in therapeutic treatments. They may alsobe used in novelty items, for example greetings cards in which a messageor design appears on exposure to light.

[0012] The barrier is suitably a polyurea or aminoplast composition.Gelatin, gum arabic, polyvinyl alcohol or other materials may be used ifdesired however.

[0013] The microcapsules may be made in known manner, for example astaught in U.S. Pat. Nos. 2,739,456, 2,800,457, 3,755,190, 3,914,511,3,796,669, 4,001,140, 4,087,376, 4,089,802 or 4,025,455.

[0014] Micro particles, e.g. microcapsules, of 1 to 50 microns averagediameter may be used to provide good continuous colour with r solutionthat is required for sharp colour writing etc.

[0015] The barrier should be robust; it should not be readily breakableby pressure or exposure to light or to a photocurable resin in which itis intended to disperse it. The barrier should remain intact in thefinished product after colour formation.

[0016] The photocurable and photocolourable composition may be made bydispersing the particles in a photocurable composition which ispreferably curable to form a transparent solid and which is preferablyfree from colour forming materials. It may, however, be desired forexample for aesthetic purposes to have a lightly coloured “background”to the image. We prefer to spray dry micro-capsules for example byevaporating the water from tiny droplets of an aqueous microcapsulesuspension, as we have found spray dried microcapsules easy to dispersein such compositions. If desired a dispersing aid may be used to assistin dispersing them in the composition. In a photocurable photocolourablecomposition containing the particles, a colour stabiliser may be presentin the particles and a longer wave-length photo-initiator may also bepresent in the resin if desired.

[0017] The light curable resin is preferably an epoxy, vinyl ether oracrylate resin or a mixture thereof. Such light curable resins aredescribed in EP 605,361 EP 360,869, U.S. Pat. No. 4,156,035, WO92/15620, EP 646,580, EP 425,441 A2, SMC 60102 and SMC 60093. Many lightcurable resins are commercially available, for example Cibatool XB5170,5180, 5190, Somos 6110, 7110, RP cure 100 HC, Exactomer 2202 SF, HTG 324and Stareocol H-N 9000 (He—Cd laser), Somos 6100, 7100, Cibatool SL5410, 5180, Exactomer HTG 35X, and RP cure 100 AR (Argon ion laser) andCibatool SL 5510, 5190, SL 5195 (YAG laser).

[0018] The photo-initiator, which is convertible by a photochemicalreaction into a developer, is preferably an acylphosphine oxide orsulphide and/or a compound which generates acid when irradiated.

[0019] The compound which generates acid when irradiated is preferablyan onium salt, a latent sulphonic acid, a halomethyl-s-triazine, ormetallocene or a chlorinated acetophenone or a benzoin phenyl ether.

[0020] Preferred onium salt photoiniators are aryl diazonium,diaryliodonium; triaryl sulphonium, triaryl selenonium, dialkyl phenacylsulphonium, triaryl sulphoxonium, aryloxydiaryl sulphoxonium anddialkylphenacyl sulphoxonium salts (especially their salts with BF₄ ⁻,PF₆ ⁻, AsF₆ ⁻ or SbF₆ ⁻), more preferably the diaryliodonium and triarylsulphonium salts which are relatively easy to prepare on a commercialscale.

[0021] The latent sulphonic acid is a compound which produces asulphonic acid on irradiation with light. Preferred latent sulphonicacids are the α-sulphonyloxy ketones. e.g. benzoin tosylate,4′-methylthio-2-(p-tosyloxy) propiophenone, α-toluene sulphonyloxypropiophenone; α-hydroxymethylb nzoin sulphonates, e.g. the methanesulphonate and p-toluene sulphonate of α-hydroxymethyl benzoin;nitrobenzyl esters of sulphonic acids, .g. 4-nitrobenzyl tosylate, 2,4-and 2,6-dinitrobenzyl tosylate,p-nitrobenzyl-9,10-diethoxyanthracene-2-sulphonate; aryldiazidonaphthaquinone-4-sulphonates; 4′-Nitrobenzyl2,4,6-triisopropylbenzenesulphone, α-sulphonyl acetophenones, e.g.α-toluene sulphonyl acetophenone and 2-methyl-2-(4-methylphenylsulphonyl)-1-phenylpropane; methane sulphonate esters of 2-hydroxy- and2,4-dihydroxybenzophenone; and1,2,3,4-tetrahydro-1-naphthylideneimino-p-toluene sulphonate.

[0022] Preferred halo methyl-s-triazines are the 2-aryl-4,6-bischloromethyl-s-triazines and preferred chlorinated acetophenones include4-tert-butyl-α,α,α,-trichloroaceto-phenone and4-phenoxy-α,α-bis-dichloroacetophenone.

[0023] A preferred metallocene is(cyclopentadi-1-enyl)[(1,2,3,4,5,6-n)-(1-methylethyl)benzene]-iron(1+)-hexafluorophosphate (1−), available from Ciba Geigy 261.

[0024] The compound which forms colour or changes colour on contact witha photochemically generated developer is preferably a triaryl methane-,diphenyl methane-, thiazine-, spiro-, lactam- or fluoran-based colourformer. Examples of Trarylmethane-based colour formers include,3-3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide,3,3-bis(p-dimethylaminophenyl)phthalide,3-(p-dimethylaminophenyl)-3-(1,2-dimethylindole-3-yl)phthalide,3-(p-dimethylaminophenyl)-3-(2-methylindole-3-yl)phthalide,3,3-bis(1,2-dimethylindole-3-yl)-5-dimethylaminophthalide,3,3-bis(1,2-dimethylindole-3-yl)-6-dimethylaminophthalide,3,3-bis(9-ethylcarbazole-3-yl)-6-dimethylaminophthalide,3,3-bis(2-phenylindole-3-yl)-6-dimethylaminophthalide,3-p-dimethylaminophenyl-3-(1-methylpyrrole-3-yl)-6-dimethylaminophthalide,etc., especially triphenyl methanes e.g. Crystal Violet Lactone.

[0025] Diphenylmethane-based colour formers include4,4′-bis-dimethylaminobenzhydryl benzyl ether,N-halophenyl-leucoauramine and N-2,4,5-trichlorophenyl-leucoauramine.

[0026] Thiazine-based colour formers include benzoyl-leucomethylene blueand p-nitrobenzoyl-leucomethylene blue.

[0027] Spiro-based colour formers include 3-methyl-spiro-dinaphthopyran,3-ethyl-spiro-dinaphthopyran, 3-phenyl-spirodinapthopyran,3-benzyl-spiro-dinaphthopyran,3-methyl-naphtho-(6′-methoxybenzo)spiropyran and3-propyl-spiro-dibenzopyran.

[0028] Lactam-based colour formers include rhodamine-b-anilinolactam,rhodamine-(p-nitroanilino)lactam and rhodamine-(o-chloroanilino)lactam.

[0029] Fluoran-based colour formers include 3,6-dimethoxyfluoran,3,6-diethoxyfluoran, 3,6-dibutoxyfluoran,3-dimethylamino-7-methoxyfluoran, 3-dimethylamino-6-methoxylfluoran,3-dimethylamino-7-methoxyfluoran, 3-diethylamino-7-chlorofluoran,3-diethylamino-6-methyl-7-chlorofluoran,3-diethylamino-6,7-dimethylfuoran,3-(N-ethyl-p-toluidino)-7-methylfluoran,3-diethylamino-7-(N-acetyl-N-methylamino)fluoran,3-diethylamino-7-N-methylaminofluoran,3-diethylamino-7-dibenzylaminofluoran,3-diethylamino-5-methyl-7-dibenzylaminofluoran,3-diethylamino-7-(N-methyl-N-benzylamino)fluoran,3-diethylamino-7-(N-chloroethyl-N-methylamino)fluoran,3-diethylamino-7-diethylaminofluoran,3-(N-ethyl-p-toluidino)-6-methyl-7-phenylaminofluoran,3-(N-ethyl-p-toluidino)-6-methyl-7-phenylaminofluoran,3-diethylamino-7-(2-carbomethoxy-phenylamino)fluoran,3-(N-ethyl-N-isoamylamino)-6-methyl-7-phenylaminofluoran,3-(N-cyclohexyl-N-methylamino)-6-methyl-7-phenylaminofluoran,3-pyrrolidino-6-methyl-7-phenylaminofluoran,3-piperidino-6-methyl-7-phenylaminofluoran,3-diethylamino-6-methyl-7-xylidinofluoran,3-diethylamino-7-(o-chlorophenylamino)fluoran,3-dibutylamino-7-(o-chlorophenylamino)fluoran and3-pyrrolidino-6-methyl-7-p-butylphenylaminofluoran.

[0030] Colour formers permitting the production of a wide range ofcolours are known and have been described, for example, by Peter Gregoryin High-Technology Applications of Organic Colorants, Plenum Press,pages 124-134.

[0031] The latent sulphonic acid 4′-nitrobenzyl2,4,6-triisopropyl-benzenesulphonate[4NO₂-C₆H₄CH₂OSO₂-(2,4,6Me₂CH—)C₆H₂] may be prepared by reacting4-nitrobenzyl-alcohol with triisopropylbenzenesulphonyl chloride in thepresence of dicyclohexylamine.

[0032] The mechanical properties of the polymeric layer or threedimensional article may also be improved by irradiation with light whichdoes not cause coloration, e.g. light of high (i.e. long) UV wavelength.If desired a photoinitiator which does not generate acid when irradiatedmay be included which absorbs the high UV wavelength light, therebyfacilitating a “post cure” using high UV wavelength light withoutunwanted colour formation. This post cure can be performed on thepolymeric layer or three dimensional article using an appropriate U.V.oven.

[0033] The photo-curable, photo-colourable composition preferablycompromise

[0034] (a) 100 parts in total of photo-curable (cationic or freeradically initiated) resin;

[0035] (b) 0.01 to 5 parts microencapsulated colorants;

[0036] (c) 0 to 5 parts longer wavelength photo-initiators and orperoxides;

[0037] (d) 0 to 5 parts additives such as dispersing aid.

[0038] Microencapsulated compositions with different colour formersystem are suggested for the use in a photocurable, photocolourablecomposition to provide multicolour image. The second colour may bedeveloped at different energy wave-length to that used for the firstcolour and so on. In the present case the necessary dose of light from asingle light source can be varied by varying the amount of a UV barrieragent coencapsulated with the colour former and colour developer and/orby varying the type of coencapsulated colour developer with a givencolour former. Alternatively the energy may be varied by using differentlight sources.

[0039] The process for forming a three-dimensional article preferablyuses a stereolithography apparatus, for example the SLA 250, 350, 500,3500, 5000 supplied by 3D-Systems or the Stereos 300, 400 and 600supplied by EOS.

[0040] There is no particular limit on what the thre dimensional articlehaving selectively coloured regions can be, for example one may use theprocess to form ornamental and industrial articles and models of plantand animal parts (e.g. human body parts). Industrial articles includemechanical parts, especially those used in automobil s. Animal partsinclude bones, organs, tissues and combinations thereof. Examples ofbones include joints (e.g. ball and socket joints such as the hip andshoulder, hinge joints such as the knee and elbow) the skull, jaw,spine, ribs, collarbone, shoulder blade, humerus, radius, ulna, teeth,finger and hand bones, breast bone, femur, tibia and fibula. Examples oforgans include the liver, heart, lungs, kidneys, bladder, brain, eyes,intestines, pancreas and reproductive organs. Examples of tissue includemuscle and cartilage. The process is particularly useful for producingselectively coloured models of animal parts and these can be used by atechnician to assess the extent of certain illnesses and otherdisorders, or as a model for a surgeon to practice on before beginningsurgery. For example, models of cancerous body parts can be producedwherein cancerous cells are coloured differently from healthy tissue. Asurgeon can then practice or plan surgery using the model beforebeginning work on the patient.

[0041] As desired the three dimensional article having selectivelycoloured regions can be a model which is the same size, smaller orlarger than the original article. Selectively coloured models which arelarger than the original article are particularly useful for viewingsmall complicated internal features.

[0042] In a further embodiment the process comprises the further step ofcoating the polymeric layer or three dimensional article resulting fromthe present process with a visually transparent layer which absorbs thewavelength of light used in step b). This has the advantage of filteringout any light which could generate further unwanted colour, e.g.sunlight. A still further advantage is that the coating providesimproved gloss and transparency and reduces or eliminates the“staircase” effect seen on three dimensional models prepared in alayer-wise manner.

[0043] The visually transparent coating preferably absorbs ultraviolet(UV) light. Materials used to prepare such coatings can be made bydissolving a UV absorbing compound in a mixture comprising a resin andone or more organic solvents, for example those UV absorbing compoundsused in protective sun creams or Tinuvin 1135 and 400 available fromCiba Geigy, Bayer 325 and 340 available from Bayer PLC, and UV.Titangrade P370 and L530 available from Kemira of Finland. Suitable resinsare thermoplastic acrylic resins, e.g. Neocryl B700 and B731 from ZenecaLimited.

[0044] The visually transparent coating is preferably applied to thepolymeric layer or three dimensional article by dip or spray coating.With dip coating the depth of layer may be controlled by alteringviscosity of the material used, with layer depth increasing withviscosity.

EXAMPLE 1

[0045] Preparation of a Microencapsulated Colorant Composition DBS97/47

[0046] An oil phase was made by mixing together 1.15 parts butylatedurea formaldehyde resin (Beetle-80 ex Cytec), 0.75 parts pentaerythritoltetra (3-mercaptopropionate ex Aldrich), 0.2 parts colour former3,3-(Bis(1-octyl-2-,methylindol-3yl) phthalide (Pergascript Red 1-6B exCiba Giegy) and 0.1 parts colour developer 4-nitrobenzyltosylate (NB-201ex Midori Kagfaku Co Ltd) and 7.8 parts solvent (Solvesso-200 ex Exxon).

[0047] The oil phase was added at room temperature to a stirred (3000rpm) solution at pH 9.1 containing 0.4 parts of sulphonated colloidstabiliser (Lomar-D ex Henkel), 0.02 parts and sulphonated alkylnaphthalene (Petro-BAF ex Witco) in 19.6 parts of distilled water. Thestirring rate was increased to 6000 rpm for 5 minutes. The pH was thenreduced to 1.9 by the addition of dilute HCl. The emulsion was stirredat 100 rpm and the temperature was raised to 50° C. Stirring wascontinued at this temperature for 3 hours affording red micro-capsuleswith an average particle size of about 11 microns. The red colorationwas removed by the drop-wise addition of ammonia.

[0048] The capsule suspension was diluted by about two to three timeswith water and was forced by nitrogen through the nozzle head of a Model190 Buchi Mini Spray Dryer Unit with the inlet at about 140° C. and theoutlet at about 70° C. A colourless powder was obtained in 45% yield. Ina similar manner to above a number of different micro-encapsulatedcolorant compositions were produced with the different level and type ofcolour developers as given in Table 1. TABLE 1 Selected encapsulatedcolorant compositions Ratio of CF:CD Level of CF Samples Ref. ColourDeveloper (CD) (%) in Caps DBS97/47 4-nitrobenzyl tosylate 01:01 2% W/WNB 201 DBS97/77 Zeneca TIBO2 01:01 2% W/W DBS97/78 Zeneca TIBO2 01:02 2%W/W

EXAMPLE 2

[0049] Effect of Micro-Capsules on Rheological and Mechanical Propertiesof Base Resin

[0050] Using commercial acrylate and epoxy resin various formulationswere prepared by adding colourl ss micro-encapsulated colorantcomposition and mixing with a vibro mixer for 2 hours. The resultedformulation was a clear liquid with well dispersed microcapsuleparticles in the resin.

[0051] For selected photopolymer resins viscosity was measured using aBrookfield viscometer with controlled temperature and spindle number 27at 100 rpm. Table 2 gives the comparison with base resin measured on thesame day.

[0052] For mechanical properties measurements, 1 mm thick tensiledumbbell samples were prepared and measured as given in ASTM D630.Samples were cured using a UV oven (“Parker” 300 w/in “H” lamp) at cureenergy of 2.57 and 5.44 J/cm², as shown in Table 2 below. No significantchange in mechanical properties or viscosity was found due to additionof microcapsules to photopolymer resin. TABLE 2 Viscosity TensileElongation Modulus Strength (%) Energy Viscosity (MPa) (MPa) (J/cm²)Sample (CPS) 2.57 5.44 2.57 5.44 2.57 5.44 Commercial 190 1094 1059 60.449.8 9.7 8.0 Epoxy Cibatool XB 5170 AHPCE471 195 943 1095 54.6 57.6 9.38.2 Cibatool XB 5170 + 1% DBS 97/47 Commercial 1040 783 882 27.2 34.44.4 5.2 Acrylate Stereocol H-N 9000 AHPZP471 Stereocol H-N 1150 736 85335.2 37.9 6.7 6.0 9000 + 1% DBS97/47

EXAMPLE 3

[0053] Colourability of Formulated Resins with Encapsulated Colorants

[0054] Colourability of selected colourable resin formulationscontaining encapsulated colorants is given in Table 3. Colour wasmeasured on 2 dimensional single layer film produced by astereolithography machine (SLA 250, ex 3D Systems, USA) fitted with 25°mw He—Cd laser and further post cured with a UV lamp Philips TLK 40W/05.Colour was measured using Minolta CR200 Chroma Meter by standard CIE(Commission Intematinale de l'Eclairage) 1976, L*a*b method. TABLE 3Colourability of commercial epoxy resin (Cibatool XB 5170) formulatedwith microcapsul s with laser and postcur lamp He-Cd Laser Min. Max.Colour Colour UV Lamp Sample Low Energy High Energy Max. ColourColourable Epoxy (red units) (red units) (red units) Cibatool XB 5170 +1% w/w 2.1 13.6 22.6 DBS97/47 Cibatool XB 5170 + 1% w/w 3.5 10 23.6DBS97/77 Cibatool XB 5170 + 2% w/w 6.2 18.2 41.8 DBS97/77 Cibatool XB5170 + 1% w/w 4.9 12.2 25.8 DBS97/78

[0055] By “low energy” is meant the energy used to cure the resin, by“high energy” is meant five times the low energy and the UV lamp energywas sufficient to produce full development of the colour.

[0056] The difference in colour achieved at low and high laser powerindicates that the encapsulated colorants are suitable for the use inepoxy resin to give selective coloration. As shown for two laserenergies, the optical density of colour formed increases with theincrease in the energy dose with the same He—Cd laser at 325 nm output.The maximum colour achieved with the UV lamp suggest that colouring twodimensional film with lamp is possible and that the amount of opticaldensity of colour achieved further increases with additional energy.

EXAMPLE 4

[0057] Concentration Effect of Microcapsules DBS97/77 on the CureProperties of Resin

[0058] A photocurable, photocurable composition was prepared by addingand mixing microcapsules DBS97/77 in a commercial epoxy resin CibatoolXB 5170 (available from 3 D Systems) at 1 and 2% w.w of Cibatool resin.The formulation was loaded into the vat of a standard Stereolithographymachine fitted with He—Cd laser (SLA 250 supplied by 3 D System, USA).The resin parameters were measured by building standard window panesingle layer parts and calculating Ec and Dp as given in Table 4.

[0059] Ec is critical energy at which polymer is at “gel point”corresponding to the transition from liquid to th solid phase. Dp is the“penetration depth” of the resin, defined as the depth of resin whichresults in a reduction in the irradiance to a level equal to 1/e of thesurface irradiance, where e=2.718 . . . , the base of naturallogarithms. Single polymeric layers were prepared using diff rent scanspeed to vary the UV dose. It was found that below a UV dose of 80mJ/cm² the polymeric layer was essentially colourless to the naked eyeand above this dose the layer became noticeably red, with the intensityof colour increasing with energy dose using the same He—Cd laser at 325nm output. Therefore the composition can be used to prepare threedimensional articles having selectively coloured regions using the lowdose to cure uncoloured regions and the high dose to colour and cureselected coloured regions. TABLE 4 SLA 250 He-Cd laser data forformulation with 1 and 2% w/w microcapsule DBS97/77 Sample Ref Ec(mJ/cm²) Dp (mm) Cibatool XB 5170 13.5 0.122 Cibatool XB 5170 + 1%DBS97/77 20.6 0.114 Cibatool XB 5170 + 2% DBS97/77 23.9 0.102

EXAMPLE 5

[0060] Example 2 was repeated, except that the microcapsules DBS97/77were dispersed in commercial acrylate resin Stereocol H-N 9000(available from ZENECA) at 1% w/w resin. The photocurable,photocolourable acrylate resin sample was tested similarly as in Example4 using a stereolithography machine (SLA 250) equipped with He—Cd laser.The processing parameters were Ec 2.8 mJ/cm² and Dp 0.086 mm.

EXAMPLE 6

[0061] Evaluation of Zeneca Synthethised Colour Developer vs CommercialColour Developer

[0062] A sample containing a commercial epoxy resin and microcapsuleswas prepared as in Example 2. The resulting photocurable,photocolourable epoxy sample was tested in a stereolithography machine(SL 250) fitted He—Cd laser at 325 nm. The effect of different colourdevelopers in microcapsule compositions on the cure properties of theformulated resin was measured and is shown in Table 5. TABLE 5 Sample Ec(mJ/cm²) Dp (mm) Cibatool XB 5170 13.5 0.122 Cibatool XB 5170 +microcapsules DBS 97/47 29.0 0.114 Cibatool XB 5170 + microcapsulesDBS97/77 20.6 0.114

[0063] Zeneca colour developer has less effect on resin cure properties(laser processing parameters Ec and Op) when microcapsules containingthis colour developer is added to the base resin.

1 A process for producing a polymeric layer having a desired imagethereon or a three dimensional article comprising a number of suchlayers in which a layer of a liquid photocurable composition whichcomprises photo colourable particles is cured by light and selectedareas thereof are irradiated with light of a different dose, therebyforming the desired image which composition comprises particlesdispersed in it which are micro-capsules containing a photosensitivecolour changing composition within a barrier layer which issubstantially impermeable to the components of the colour changingcomposition or are solid particles comprising an immobilisedphotosensitive colour changing composition. 2 A process as in claim 1 inwhich the composition is both cured and coloured by cationic initiators.3 A process as claimed in claim 1 or 2 in which the colour changingcomposition comprises a photo initiator and a colour former, and inwhich the initiator is sensitive to the light used for curing and inwhich the particles comprise a light absorbing substance which absorbsat least part of the light whereby initiation of the full colour changerequires a greater dose of light in intensity or duration than is neededfor curing. 4 A process as claimed in claim 3 in which the lightabsorbing substance is present in the barrier of the micro-capsule. 5 Aprocess as claimed in any preceding claim in which a cationic initiatoris used as a photo initiator and the particles comprise an alkalicapable of neutralising cations initially produced in the initiatingprocess. 6 A process as claimed in any preceding claim which comprisesforming a stereo lithographic image. 7 Particles for carrying out aprocess as claimed in any of claims 1 to 6 which are micro-capsulescontaining a photosensitive colour changing composition Within a barrierlayer which is substantially impermeable to the components of the colourchanging composition and which is sufficiently robust to resist rupturewhen the particles are mixed with an epoxy or vinyl ether or acrylicresin and subjected to photo initiation in the said process or which aresolid particles comprising an immobilised photosensitive colour changingcomposition. 8 Particles as claimed in claim 7 in which the photosensitive colour changing composition is in a liquid phase which issurrounded by a containing wall. 9 Particles as claim d in claims 7 or 8which are of 1 to 50 microns average diameter. 10 Particles as claimedin claims 7, 8 or 9 in which the total wall thickness comprises 15 to50% of the diameter of the particle. 11 A particle as claimed in any ofclaims 7 to 10 which comprises a cationic initiator and an alkalinesubstance in a quantity sufficient to neutralise some but not all of thecations producible by the initiators whereby the particles aremaintained in an unchanged state of colour until subjected to aninitiating dose of light. 12 A particle as claimed in any of claims 7,8, 9 or 10 which comprises an aminoplast wall. 13 A process of producingparticles as claimed in any of claims 7 to 12 which comprises spraydrying a dispersion of the particles in an aqueous medium. 14 A processof producing a particle as claimed in any of claims 7 to 12 whichcomprises contacting the particle with ammonia to decolourise and/orprovide a reserve alkalinity in the particle. 15 A micro-capsule asclaimed in any preceding claim in which the barrier wall is sufficientlyrobust to remain intact in the finished product after colour formation.16 A particle as claimed in any of claims 7 to 15 which comprises acolour stabiliser. 17 A process or particle as claimed in any precedingclaim which comprises an ultra violet absorber. 18 A particle as claimedin claim 17 in which the ultra violet absorber is a component of thewall of a micro-capsule. 19 A composition for carrying out a process asclaimed in any of claims 1 to 6 which comprises a photocurable resin andparticles as claimed in any one of claims 7 to 18 dispersed in saidresin. 20 A Composition for carrying out the process as claimed in anyof claims 1 to 6 or which comprises particles as claimed in any ofclaims 7 to 18 in which different particles have different colour formersystems thereby providing a multi-colour image in a final product, inwhich each colour is developed at a different energy to those used forother colours. 21 A composition as claimed in claim 19 in which a photoinitiator which does not generate acid when irradiated is included, thesaid photo initiator being sensitive to high ultra violet wavelengthlight and facilitating a post cure of the resin without unwanted colourformation. 22 A product produced by a process as claimed in any ofclaims 1 to 6 containing particles as claimed in any of claims 7 to 18or produced using a composition as claimed in claims 19 or 20 whichcomprises a surrounding visually transparent layer which absorbs thelight capable of generating further colour change.